Nitride semiconductor substrate production method thereof and semiconductor optical device using the same

ABSTRACT

Disclosed are a nitride semiconductor substrate and a production method thereof. Seed crystals made of GaN or AlGaN with a relatively low AlN molar fraction is selectively grown on a first group-III nitride semiconductor, such as GaN, to have a specific crystal face. Then, on the seed crystals, an AlGaN with a high AlN molar fraction is grown through a second group-III nitride semiconductor, such as AlN deposited at a low temperature. The present invention can provide an AlGaN-crystal substrate having a low dislocation density in a wide area without any crack, and a high-performance short-wavelength optical device using the substrate.

TECHNICAL FIELD

The present invention relates to the structure and production method ofa group-III nitride semiconductor substrate which is expected to beapplied, particularly, in the field of optical information processing.The present invention also relates to an optical semiconductor deviceformed using such a substrate.

BACKGROUND ART

Heretofore, there has been known light-emitting and light-receivingdevices with a multilayered structure having a base layer made ofgroup-III nitride semiconductor based on GaN or AlGaN with a relativelylow molar fraction of AlN (see, for example, the after-mentioned PatentPublication 1 or Non-Patent Publication 1). FIG. 8 shows the structureof crystal layers in a typical conventional GaN-based light-emittingdiode. This light-emitting diode includes n-GaN layer 103, an n-GaNfirst cladding layer 104, a GaInN/GaN multi-quantum-well active layer105, a p-AlGaN cap layer 106, a p-GaN second cladding layer 107 and ap-GaN contact layer 108, which are deposited in this order on a sapphiresubstrate 101 through a low-temperature buffer layer 102 made of GaN orAIN.

For example, the GaInN/GaN multi-quantum-well active layer 105 is formedby sandwiching a GaInN quantum-well layer with 3 nm thickness betweenGaN barrier layers each with 10 nm thickness. After crystal growth, anohmic semitransparent electrode 109 and a bonding pad electrode 110 eachformed of a metal thin film are formed on the surface of the p-GaNcontact layer 108, and an n-type electrode 111 is formed on a portion ofthe surface of the n-GaN first cladding layer 104 which is exposedoutside through an etch process. The molar fraction of AlN in the GaInNquantum-well layer can be selectively changed to produce variouslight-emitting diodes having a wavelength in the range of about 380 to620 nm.

While a semiconductor laser can be produced in a similar structure, awavelength range capable of generating laser oscillation at roomtemperature is narrower than that in the light-emitting diodes.

It is known that the nitride-semiconductor crystal layers produced inthe above manner include threading dislocations at a density of 10⁸ cm⁻²or more, which act as non-radiative centers. Thus, the high threadingdislocation density causes deterioration in the external quantumefficiency of the light-emitting diode, and increase in the thresholdcurrent or deterioration in the element lifetime of the semiconductorlaser. It is also known that threading dislocations in a photodetector,such as photodiodes, cause increase in dark current. Thus, inphotodetectors, it is also regarded as one essential challenge toachieve a reduced threading dislocation density.

In late years, an epitaxial lateral overgrowth (ELO) method has beenincreasingly used as one technique for obtaining a reduced threadingdislocation density. FIG. 9 shows the structure of a low-dislocation GaNsubstrate produced using the ELO method. In FIG. 9, a GaN layer 203 isgrown on a sapphire substrate 201 through a low-temperature buffer layer202 made of GaN or AlN. Equally-spaced stripe masks 204 made, forexample, of SiO₂ is formed on the surface of the GaN layer 203, and thena GaN overgrowth layer 205 is grown on the surfaces of the GaN layer 203and the stripe masks 204. The crystal growth of the GaN overgrowth layer205 is initiated only in portions of the surface of the GaN layer 203which are not covered by the stripe masks 204, or in exposed portions ofthe surface of the GaN layer 203, and then crystals grow in the lateraldirection to cover over the surfaces of the stripe masks 204 in a while.Finally, the GaN overgrowth layer 205 is formed as a film having a flatsurface as shown in FIG. 9.

In the above crystal growth process of the GaN overgrowth layer 205,dislocations 206 to be essentially threaded perpendicular to the crystalgrowth direction almost never exist above the stripe masks 204 exceptfor crystal junction areas 207. Thus, in the GaN overgrowth layer 205,areas having an extremely low threading dislocation density of about 10⁵to 10⁷ cm⁻² are formed above the stripe masks 204 except for the areasbetween the adjacent the stripe masks 204. This substrate can be used toproduce a light-emitting diode or semiconductor laser reduced innon-radiative recombination centers to provide high efficiency andexcellent characteristics. A photodetector produced by formingphotodetector elements on the low dislocation area of the substrate canhave low dark current reduced by several digits.

Lately, an AlGaN-based ultraviolet light-emitting diode grown on a bulkGaN substrate has been reported (see the after-mentioned Non-PatentPublication 2). A 305 nm ultraviolet light-emitting diode using AlInGaNmulti-quantum-wells has also been reported (see the after-mentionedNon-Patent Publication 3).

[Patent Publication 1]

Japanese Patent Laid-Open Publication No. 2001-44497

[Non-Patent Publication 1]

Hiroshi Amano, et al., “Low-temperature Deposited Layer in Group-IIINitride Semiconductor Growth on Sapphire Substrate”, Journal of SurfaceScience Society of Japan, 2000, Vol. 21, No. 3, pp 126-133

[Non-Patent Publication 2]

Toshio Nishida, et al., “Efficient and high-power AlGaN-basedultraviolet light-emitting diode grown on bulk GaN”, APPLIED PHYSICSLETTERS, American Institute of Physics, 6 AUG. 2001, Vol. 79, No. 6, pp711-712 [Non-Patent Publication 2]

Muhammad Asif KHAN, et al., “Stripe Geometry Ultraviolet Light-EmittingDiodes at 305 Nanometers Using Quaternary AlInGaN Multiple QuantumWells”, The Japan Society of Applied Physics, Jpn, J. Appl. Phys., 1Dec. 2001, Vol. 40, Part 2, No. 12A, pp. L1308-L1310

DISCLOSURE OF INVENTION

Methods using the conventional dislocation-reducing technique havedifficulties in applications to light-emitting devices to be operated ina short wavelength range of 370 nm or less. Because, in order to obtainsuch short-wavelength light-emitting devices, such methods are requitedto use an AlGaN cladding layer having a high AlN molar fraction so as toassure a bandgap difference for confining carriers in an active layer.In case of semiconductor lasers, it is essentially required to use anAlGaN cladding layer having a higher AlN molar fraction and a largerfilm thickness in view of an additional requirement on the lightconfinement necessary for laser emission.

However, in the process of growing such an AlGaN cladding layer having ahigh AlN molar fraction on a GaN layer, a crack occurs in the AlGaNcladding layer. Specifically, a tensile stress is generated in the AlGaNcladding layer due to the lattice mismatching with the GaN layer, andwhen the AlGaN cladding layer is grown to have a film thickness greaterthan a critical value, the crack is formed therein to release thestress. For example, while a semiconductor laser having an emissionwavelength of 350 nm can be obtained by using an AlGaN cladding layerhaving an AlN molar fraction of about 20%, the critical film thicknessof the AlGaN cladding layer on a GaN layer is about 0.2 μm whichinvolves an issue on device production. Because the cladding layer isrequired to have a film thickness of at least 0.4 μm in view of thecarrier and/or light confinement

As to photodetectors, the reduction in the density of threadingdislocation causing dark current is essential as described above. Thereis also the need for providing a photodetector capable of operating in awavelength range of 300 nm or less, which can be achieved only by usinga nitride semiconductor. In this case, it is required to use AlGaNhaving a higher AlN molar fraction than that in light-emitting devices.Thus, the conventional methods cannot achieve the formation of ahigh-quality crystal having a low threading dislocation density withoutany crack.

The present invention is directed to selectively grow a seed crystalmade of GaN, or AlGaN having a relatively low AlN molar fraction, on agroup-III nitride semiconductor, such as GaN, to have a specific crystalface, and then grow an AlGaN layer having a high AlN molar fractionthrough a group-III semiconductor, such as AlN deposited at a lowtemperature, so as to provide an AlGaN-crystal substrate having a lowdislocation area in a wide range without any crack, and ahigh-performance optical device.

Specifically, according to a first aspect of the present invention,there is provided a nitride semiconductor substrate for a deviceemitting a short wavelength light of 370 nm or less comprising asingle-crystal substrate, a first semiconductor layer made of (0001)face Al_(x)Ga_(1-x-y)In_(y)(0≦x≦0.1, 0≦y≦0.5), a plurality of seedcrystals each made of Al_(x)Ga_(1-x-y)In_(y)N (0≦x≦0.1, 0≦y≦0.5) andformed on the semiconductor layer to have a slant surface while beingarranged in an equally-spaced stripe pattern, an intermediate layer madeof Al_(a)Ga_(1-a-b)In_(b)N (0.1≦a≦1, 0≦b≦1) and deposited on the wholesurface of the seed crystals at low temperature of 300 to 800° C., and asecond semiconductor layer made of Al_(a)Ga_(1-a-b)In_(b)N (0.1≦a≦1,0≦b≦1) and formed on the intermediate layer to have a flat surface.

According to a second aspect of the present invention, there is provideda method of producing a nitride semiconductor substrate comprising thesteps of (A) performing a selective growth process to form a seedcrystal made of Al_(x)Ga_(1-x-y)In_(y)N (0≦x≦0.1, 0≦y≦0.5) on a firstsemiconductor layer made of (0001) face Al_(x)Ga_(1-x-y)In_(y)N(0≦x≦0.1, 0≦0.5), in such a manner that the seed crystals are provided,respectively, with slant surfaces, and arranged in an equally-spacedstripe pattern, (B) depositing an intermediate layer made ofAl_(a)Ga_(1-a-b)In_(b)N (0.1≦a≦1, 0≦b≦1), on the seed crystals at lowtemperature of 300 to 800° C., and (C) growing a second semiconductorlayer made of Al_(a)Ga_(1-a-b)In_(b)N (0.1≦a≦1, 0≦b≦1), on theintermediate layer until the second semiconductor layer has a flatsurface.

According to a third aspect of the present invention, there is provideda nitride semiconductor substrate comprising a single-crystal substratehaving equally-spaced stripe-shaped grooves, a plurality of seedcrystals each made of Al_(x)Ga_(1-x-y)In_(y)N (0≦x≦0.1, 0≦y≦0.5) andformed on the convex portions of the single-crystal substrate to have aslant surface portion, an intermediate layer made ofAl_(a)Ga_(1-a-b)In_(b)N (0.1≦a≦1, 0≦b≦1) and deposited on the seedcrystals at low temperature of 300 to 800° C., and a secondsemiconductor layer made of Al_(a)Ga_(1-a-b)In_(b)N (0.1≦a≦1, 0≦b≦1) andformed on the intermediate layer to have a flat surface.

According to a fourth aspect of the present invention, there is provideda method of producing a nitride semiconductor substrate comprising thesteps of (A) forming a plurality of seed crystals each made ofAl_(x)Ga_(1-x-y)In_(y)N (0≦x≦0.1, 0≦y≦0.5), on a single-crystalsubstrate having equally-spaced stripe-shaped grooves, in anequally-spaced stripe pattern, (B) depositing an intermediate layer madeof Al_(a)Ga_(1-a-b)In_(b)N (0.1≦a≦1, 0≦b≦1), on the seed crystals at lowtemperature of 300 to 800° C., and (C) growing a semiconductor layermade of Al_(a)Ga_(1-a-b)In_(b)N (0.1≦a≦1, 0≦b≦1), on the intermediatelayer until the semiconductor layer has a flat surface.

According to a fifth aspect of the present invention, there is provideda semiconductor light-emitting device comprising the nitridesemiconductor substrate set forth in the first or third aspect of thepresent invention, a first conductive-type cladding layer made ofAl_(x)Ga_(1-x-y)In_(y)N (0.1≦x≦1, 0≦y≦0.5), a second conductive-typecladding layer made of Al_(x)Ga_(1-x-y)In_(y)N (0.1≦x≦1, 0≦y≦0.5), andan active layer made of Al_(x)Ga_(1-x-y)In_(y)N (0≦x≦1, 0≦y≦1) andhaving a bandgap less than that of the cladding layers.

The above semiconductor light-emitting device may have an emissionwavelength of 370 nm or less, and an external quantum efficiency (ηext)of 0.1% or more, wherein the external quantum efficiency(ηext)=Po/(I×V), wherein Po is a light output, I being an operatingcurrent of the device, and V being an operating voltage of the device.

According to a sixth aspect of the present invention, there is provideda photodetector comprising the nitride semiconductor substrate set forthin the first or third aspect of the present invention, a firstconductive-type semiconductor layer made of Al_(x)Ga_(1-x-y)In_(y)N(0.1≦x≦1, 0≦y≦0.5), a second conductive-type semiconductor layer made ofAl_(x)Ga_(1-x-y)In_(y)N (0.1≦x≦1, 0≦y≦0.5), and a light-absorbing layermade of Al_(x)Ga_(1-x-y)In_(y)N (0≦x≦1, 0≦y≦1).

In the present invention, the single-crystal substrate may be made ofone selected from the group consisting of sapphire, silicon carbide,silicon and ZrB₂.

The term “slant surface” herein a surface which is not perpendicular orparallel to the surface of the single-crystal substrate, and the slantsurface may be the (1-101) face or (11-22) face of the seed crystalmade, for example, of GaN. The conditions of growing the seed crystalcan be varied to switchably create a surface perpendicular to thesurface of the single-crystal substrate or the slant surface.

While the spacing or interval in the equally-spaced stripe pattern maybe set at any value to the same effect of obtaining a reduceddislocation density, the lower limit of the interval is determined bythe resolution of a photolithography for use in patterning into theequally-spaced stripe pattern, and it is practically difficult to setthe interval at 1 μm or less. The upper limit of the interval isdependent on a time required for obtaining a flat surface in the crystalformed on the seed crystals. If the equally-spaced stripe pattern has aninterval of 20 μm, the required time will go over 10 hours, which isdisadvantageous to actual production. Thus, it is desirable to set theinterval in the range of about 1 to 20 μm.

The intermediate layer is deposited at a low temperature of 300 to 800°C. If the temperature is greater than 800° C., the intermediate layerwill be formed as a single crystal, and the lattice strain between theseed crystals, such as GaN, and the second semiconductor layer, such asAlGaN, formed on the intermediate layer will cause a crack in the secondsemiconductor layer etc. If the temperature is less than 800° C., nolayer will be deposited due to thermal decomposition of a source gas.The intermediate layer deposited in the range of 300 to 800° C. is anaggregate of fine granular crystals which have a function of relaxingthe stress due to the lattice mismatching to prevent the occurrence ofcrack.

In the present invention, the second semiconductor layer made ofAl_(x)Ga_(1-x-y)In_(y)N (0.1≦a≦1, 0≦b≦1) is formed on the intermediatelayer to have a flat surface. For example, in an optical device using anextremely thin active layer, so-called quantum well, if the surface ofthe second semiconductor layer is not flattened or is not flat, theactive layer cannot be formed in a uniform film thickness.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1(a) to 1(d) are schematic diagrams showing the structure andproduction method of a low-dislocation-density group-III nitridesemiconductor substrate according to a first embodiment of the presentinvention.

FIG. 2 is a transmission electronic micrograph of the section of thelow-dislocation-density group-III nitride semiconductor substrateaccording to the first embodiment.

FIGS. 3(a) to 3(d) are schematic diagrams showing the structure andproduction method of a low-dislocation-density group-III nitridesemiconductor substrate according to a second embodiment of the presentinvention.

FIG. 4 is a transmission electronic micrograph of the section of thelow-dislocation-density group-III nitride semiconductor substrateaccording to the second embodiment.

FIG. 5 is a schematic diagram showing the structure of an ultravioletlight-emitting diode according to a third embodiment of the presentinvention.

FIG. 6 is a schematic diagram showing the structure of an ultravioletsemiconductor laser according to a fourth embodiment of the presentinvention.

FIG. 7 is a schematic diagram showing the structure of an ultravioletlight-emitting diode according to a fifth embodiment of the presentinvention.

FIG. 8 is a schematic diagram showing the structure of crystal layers ofa conventional GaN-bases light-emitting diode.

FIG. 9 is a schematic diagram showing the structure of alow-dislocation-density GaN substrate obtained through a conventionalepitaxial lateral overgrowth (ELO) method.

FIG. 10 is a graph showing optical output characteristics obtained froman inventive example and conventional examples.

BEST MODE FOR CARRYING OUT THE INVENTION

With reference to the drawings, an embodiment of the present inventionwill now be described.

[First Embodiment]

FIGS. 1(a) to 1(d) show the structure and production method of alow-dislocation-density group-III nitride semiconductor substrateaccording to a first embodiment of the present invention. Through anorganometallic-compound vapor phase growth, an AlN low-temperaturebuffer layer 2 and a GaN layer 3 are formed on a (0001) face sapphiresubstrate 1 in this order, at about 500° C. and about 1000° C.,respectively. After the sapphire substrate 1 with the buffer and GaNlayers 1, 3 is taken out of a growth apparatus, stripe masks, forexample, comprising SiO₂ masks 4 each having a width of 5 μm andarranged an interval of 10 μm, are formed along the crystal orientation<11-21>thereof, and then the GaN layer can be selectively regrown atappropriate conditions to form a plurality of GaN seed crystals 5 eachhaving a specific shape as shown in FIG. 1(b). Each of the formed GaNseed crystals has a slant surface corresponding to the (1-101) face.

After the SiO₂ masks 4 or stripe masks are removed, the substratetemperature is reduced down, for example, to 500° C., to grow an AlNlow-temperature-deposited intermediate layer 6, as shown in FIG 1(c).Then, an Al_(0.2)Ga_(0.8) ₈N layer 7 is grown at about 1000° C., asshown in FIG. 1(d). The Al_(0.2)Ga_(0.8)N layer 7 is flattened alongwith the growth, and finally has a fully flat surface. In theAl_(0.2)Ga_(0.8)N layer 7 formed in this way, most dislocations 9 areconcentrated only around respective crystal junction areas 8 or therespective approximate centers between the adjacent GaN seed crystals 5,and extremely limited dislocations 9 are created in the remaining area.Thus, the above method can achieve a high-quality AlGaN substrate havinga low dislocation density over the entire surface without any crack.

FIG. 2 is a transmission electronic micrograph of the section of theAlGaN substrate. Typically, the GaN layer 3 grown on the sapphiresubstrate 1 has threading dislocations at a density of about 10⁹ cm⁻².While these dislocations is turned over directly to the GaN seedcrystals 5, each of the GaN seed crystals 5 grows while maintaining the(1-101) face, and thereby the dislocations are bent at an approximatelyright angle, as seen in FIG. 2.

Thus, almost no dislocation is observed in the areas above the GaN seedcrystals 5. The dislocations bent and propagated in a directionperpendicular to the growth direction are turned over to thesubsequently grown Al_(0.2)Ga_(0.8)N layer 7, and the dislocations willbe vanished in the areas where the portions of the Al_(0.2)Ga_(0.8)Nlayer 7 growing laterally from the adjacent GaN seed crystals 5 arecombined or joined together, or will be re-bent in the growth directionin these junction areas to extend toward the surface of theAl_(0.2)Ga_(0.8) N layer 7. Thus, except for the junction areas, theremaining area has an extremely low dislocation density. Thislow-dislocation area has a dislocation density of 10⁷ cm⁻². Alight-emitting device produced by forming device elements on thislow-transition-density substrate can have significantly reducednon-radiative recombination centers to obtain high quantum efficiency. Alight-receiving device produced by forming device elements on thelow-transition-density substrate can achieve excellent characteristicshaving high voltage resistance and reduced dark current.

While the dislocation-reducing technique has been described inconnection with the Al_(0.2)Ga_(0.8)N layer 7 in the first embodiment,it can also be applied to any AlGaInN having different compositions.Further, the GaN layer 3 and the GaN seed crystals 5 may be substitutedwith an AlGaN layer or AlGaInN layer. However, if the seed crystals 5have a high AlN molar fraction, the deposition of polycrystals on theSiO₂ masks 4 or stripe masks will be undesirably increased. Thus, theAlN molar fraction in AlGaN is preferably set at 10% or less.

Instead of the AlN low-temperature intermediate layer, AlGaInN formed atthe same low temperature may be used. Further, while the firstembodiment has been described in connection of the method using the SiO₂masks 4 or stripe masks along the orientation <11-20>to provide theslant surface of the seed crystal 5 corresponding to the (1-101) face,the stripe masks along the orientation <1-100>may be used to provide theslant surface of the seed crystal 5 corresponding to the (11-22) face soas to obtain the same effect. Furthermore, while the first embodimenthas employed the (0001) face sapphire substrate, it is understood thatany other suitable substrate, such as a sapphire substrate havingdifferent orientations, or a single-crystal substrate made of SiC, Si,ZrB₂ etc, may be used to obtain the same effect.

[Second Embodiment]

FIGS. 3(a) to 3(d) show the structure and production method of alow-dislocation-density group-III nitride semiconductor substrateaccording to a second embodiment of the present invention. In advance ofthe crystal growth of nitride semiconductor, equally-spaced grooveshaving, for example, a width of 5 μm, an interval 10 μm and a depth 2μm, are formed in the surface of a (0001) face sapphire substrate 11.The orientation of each of the equally-spaced grooves 12 is arranged inparallel to the crystal orientation <1-100>of the sapphire substrate 11.Then, as shown in FIGS. 3(b) and 3(c), an AlN low-temperature bufferlayer 13 and GaN seed crystals 14 are formed on the sapphire substrate11 in this order, respectively, at about 500° C. and about 1000C,through an organometallic-compound vapor phase growth. In this process,the GaN seed crystal can be grown at appropriate conditions to form the(1-101) face in a slant surface of each of the GaN seed crystals 14 onthe convex portions 15 of the sapphire substrate.

While crystals are concurrently grown in the respective grooves orconvex portions of the sapphire substrate, the seed crystals on theconvex portions 15 of the sapphire substrate are connected with eachother to block the supply of raw materials therein so as to discontinuethe growth in the convex portions. The substrate temperature is thenreduced down to about 500° C. to grow an AlN low-temperature-depositedintermediate layer 16. Then, after re-heated up to 1000° C., anAl_(0.2)Ga_(0.8)N layer 17 is grown, as shown in FIG. 3(d). TheAl_(0.2)Ga_(0.8)N layer 17 is flattened along with the growth, andfinally has a fully flat surface. In the Al0.2Ga_(0.8)N layer 17 formedin this way, most dislocations 19 are concentrated only aroundrespective crystal junction areas 18 or the respective approximatecenters between the adjacent GaN seed crystals 14, and extremely limiteddislocations 19 are created in the remaining area. Thus, the abovemethod can achieve a high-quality AlGaN substrate having a lowdislocation density over the entire surface without any crack.

FIG. 4 is a transmission electronic micrograph of the section of theAlGaN substrate. Typically, threading dislocations at a density of about10⁹ cm⁻² exist below the GaN seed layers 14 grown on the sapphiresubstrate 11. However, the dislocations 19 are bent at an approximatelyright angle, as seen in FIG. 4, because each of the GaN seed crystals 14grow while maintaining the (1-101) face. Thus, almost no dislocation 19is observed in the areas above the GaN seed crystals 14. Thedislocations bent and propagated in a direction perpendicular to thegrowth direction are turned over to the subsequently grownAl_(0.2)Ga_(0.8)N layer 17, and the dislocations will be vanished in theareas where the portions of the Al_(0.2)Ga_(0.8)N layer 17 growinglaterally from the adjacent GaN seed crystals 14 are combined or joinedtogether, or will be re-bent in the growth direction in these junctionareas to extend toward the surface of the Al_(0.2)Ga_(0.8)N layer 17.

Thus, except for the junction areas, the remaining area has an extremelylow dislocation density. This low-dislocation area has a dislocationdensity of 10⁷ cm⁻². A light-emitting device produced by forming deviceelements on this low-transition-density substrate can have significantlyreduced non-radiative recombination centers to obtain high quantumefficiency. A light-receiving device produced by forming device elementson the low-transition-density substrate can achieve excellentcharacteristics having high voltage resistance and reduced dark current.

While the dislocation-reducing technique has been described inconnection with the Al_(0.2 Ga) _(0.8)N layer 17 in the secondembodiment, it can also be applied to any AlGaInN having differentcompositions. Further, the GaN seed crystals 14 may be substituted withan AlGaInN layer. The low-temperature intermediate layer may also besubstituted with AlGaInN. Further, while the first embodiment has beendescribed in connection of the method using the equally-spaced grooves12 along the orientation <11-20>to provide the slant surface of the seedcrystal 14 corresponding to the (1-101) face, the equally-spaced grooves12 along the orientation <1-100>may be used to provide the slant surfaceof the seed crystal 14 corresponding to the (11-22) face so as to obtainthe same effect. Furthermore, while the second embodiment has employedthe (0001) face sapphire substrate, it is understood that any othersuitable substrate, such as a sapphire substrate having differentorientations, or a single-crystal substrate made of SiC, Si, ZrB₂ etc,may be used to obtain the same effect.

[Third Embodiment]

FIG. 5 shows the structure of an ultraviolet light-emitting diodeaccording to a third embodiment of the present invention. After formingthe low-transition-density nitride semiconductor substrate illustratedin FIG. 1, an n-Al_(0.2)Ga_(0.8)N first cladding layer 21, amulti-quantum-well active layer 22, a p-Al_(0.4)Ga_(0.6)N cap layer 23,a p-Al_(0.2)Ga_(0.8)N second cladding layer 24, and a p-GaN contactlayer 25 are successively grown on the nitride semiconductor substratein this order, through an organometallic-compound vapor phase growth.

For example, the multi-quantum-well active layer 22 comprises a GaNquantum layer having a thickness of 3 nm, and an Al_(0.1)Ga_(0.8)Nbarrier layer having a thickness of 9 nm. After the completion of thecrystal growth, an ohmic semitransparent electrode 26 and a bonding padelectrode 27 each formed of a metal thin film are formed on the surfaceof the p-GaN contact layer 25, and an n-type electrode 28 is formed on aportion of the surface of the n-Al_(0.2)Ga_(0.8)N first cladding layer21 which is exposed outside through an etch process.

In this ultraviolet light-emitting diode, a certain voltage is appliedbetween the bonding pad electrode 27 and the n-type electrode 28 tosupply a current, so that electrons and electron holes are injected intothe multi-quantum-well active layer 22 to emit a light with a wavelengthof about 350 nm corresponding to a bandgap thereof. The layersconstituting the light-emitting diode are deposited on theaforementioned Al_(0.2)Ga_(8.0)N layer 7 having a low dislocationdensity. Thus, the multi-quantum-well active layer 22 also has a lowdislocation density. That is, the rate of non-radiative recombination issignificantly low, and most electron-hole pairs are recombined inconcurrence with light emissions. This can provide extremely highquantum efficiency. While the third embodiment has employed thelow-transition-density AlGaN substrate according to the firstembodiment, the low-transition-density AlGaN substrate according to thesecond embodiment may also be used to obtain the same effect.

[Fourth Embodiment]

FIG. 6 shows the structure of an ultraviolet semiconductor laseraccording to a fourth embodiment of the present invention. After formingthe low-transition-density nitride semiconductor substrate illustratedin FIG. 1, an n-Al_(0.2)Ga_(8.0)N first cladding layer 31, ann-Al_(0.1)Ga_(0.9)N first optical guide layer 32, a multi-quantum-wellactive layer 32, a p-Al_(0.4)Ga_(0.6)N cap layer 34, ap-Al_(0.1)Ga_(0.9)N second optical guide layer 35, a p-Al_(0.2)Ga_(8.0)Nsecond cladding layer 36, and a p-GaN contact layer 37 are successivelygrown on the nitride semiconductor substrate in this order, through anorganometallic-compound vapor phase growth.

For example, the multi-quantum-well active layer 33 comprises a GaNquantum layer having a thickness of 3 nm, and an Al_(0.1)Ga_(0.9)Nbarrier layer having a thickness of 9 nm. After the completion of thecrystal growth, a ridge stripe 38, for example, having a width of 2 mmis formed on the surface of the p-GaN contact layer 37, and a dielectriclayer 39 having a low refractive index and made, for example, of SiO₂,are formed on either side of the ridge stripe 38.

A p-type electrode 40 is formed on the ridge stripe 38 and thedielectric layers 39. An n-type electrode 41 is also formed on a portionof the surface of the n-Al_(0.2)Ga_(0.8)N first cladding layer 31 whichis exposed outside through an etch process. Further, through an incisionor etch process, resonant mirrors are formed on the both sides of thedevice, respectively.

In this ultraviolet semiconductor laser, a certain voltage is appliedbetween the p-type electrode 40 and the n-type electrode 41 to supply acurrent, so that electrons and electron holes are injected into themulti-quantum-well active layer 33 to emit a light with a wavelength ofabout 350 nm corresponding to a bandgap thereof and a laser beam throughan optical amplification action. The layers constituting thesemiconductor laser are deposited on the aforementionedAl_(0.2)Ga_(0.8)N layer 7 having a low dislocation density. Thus, themulti-quantum-well active layer 33 also has a low dislocation density.

That is, the rate of non-radiative recombination is significantly low,and most electron-hole pairs are recombined in concurrence with lightemissions. This can provide a high optical gain to achieve a laseroscillation with high differential efficiency and low threshold currentdensity. While the fourth embodiment has employed thelow-transition-density AlGaN substrate according to the firstembodiment, the low-transition-density AlGaN substrate according to thesecond embodiment may also be used to obtain the same effect.

[Fifth Embodiment]

FIG. 7 shows the structure of an ultraviolet photodetector according toa fifth embodiment of the present invention. While this structure issimilar to the low-dislocation-density nitride semiconductor substrateillustrated in FIG. 1, the top layer is changed from theAl_(0.2)Ga_(0.8)N layer 7 to an Al_(0.4)Ga_(0.6)N layer 7′. On thissubstrate, an n-Al_(0.4)Ga_(0.6)N layer 51 and an n⁻-Al_(0.4)Ga_(0.6)Nlight-absorbing layer 52 are successively grown in this order, throughan organometallic-compound vapor phase growth. After the completion ofthe crystal growth, a Schottky semitransparent electrode 53 and abonding pad electrode 54 each formed of a metal thin film are formed onthe surface of the n⁻-Al_(0.4)Ga_(0.6)N light-absorbing layer 52, and ann-type electrode 55 is formed on a portion of the surface of then-Al_(0.4)Ga_(0.6)N layer 51 which is exposed outside through an etchprocess.

In this ultraviolet photodetector, a certain reverse bias voltage isapplied between the bonding pad electrode 54 and the n-type electrode55, to allow a light having a wavelength of 300 nm or less to beincident through the Schottky semitransparent electrode 53. Thus, thelight is absorbed by the n⁻-Al_(0.4)Ga_(0.6)N light-absorbing layer 52below the Schottky semitransparent electrode 53 to create electron-holepairs. An electric field generated by the bias voltage acts toaccelerate these electron-hole pairs. Then, the electrons and the holesare moved, respectively, to the n-type electrode 55 and the bonding padelectrode 54, and then extracted out of the device as a photoelectriccurrent.

The layers constituting the ultraviolet photodetector are deposited onthe aforementioned Al_(0.4)Ga_(6.0)N layer 7′ having a low dislocationdensity. Thus, n⁻-Al_(0.4)Ga_(6.0)N light-absorbing layer 52 also has alow dislocation density. This can provide high quantum efficiency andsignificantly reduced dark current to achieve excellent devicecharacteristic. Particularly, the dark-current reduction effect issignificant. Specifically, the dark current for a reverse bias voltageof 10V was 10 pA/cm2 or less. This value is substantially reduced byabout eight digits as compared to the dark current in a conventionaldevice produced by forming device elements on a crystal having a highdislocation density.

While the fifth embodiment has employed the low-transition-density AlGaNsubstrate according to the first embodiment, the low-transition-densityAlGaN substrate according to the second embodiment may also be used toobtain the same effect. Further, while the fifth embodiment has shown aSchottky-type photodiode as a photodetector, the present invention canbe applied to different types of photodetector, such as a pin-typephotodiode or a phototransistor. Further, the present invention can beapplied to an image sensor, for example, integrated in 2-dimensionalarray.

EXAMPLE

Based on the structure of the device according to the third embodiment,an ultraviolet light-emitting diode was produced. On thelow-dislocation-density nitride semiconductor substrate illustrated inFIG. 1, an n-Al_(0.2)Ga_(0.8)N first cladding layer 21, amulti-quantum-well active layer 22, a p-Al_(0.4)Ga_(0.6)N cap layer 23,a p-Al_(0.2)Ga_(0.8)N second cladding layer 24, and a p-GaN contactlayer 25 were successively grown in this order, through anorganometallic-compound vapor phase growth. The multi-quantum-wellactive layer 22 was composed of a GaN quantum layer and anAl_(0.1)Ga_(0.9)N barrier layer, and the thickness of the GaN quantumlayer was adjusted to be varied in the range of 2 nm to 4 nm to changethe emission wavelength in the range of 320 nm to 360 nm. After thecompletion of the crystal growth, an ohmic semitransparent electrode 26and a bonding pad electrode 27 each formed of a metal thin film wereformed on the surface of the p-GaN contact layer 25, and an n-typeelectrode 28 was formed on a portion of the surface of then-Al_(0.2)Ga_(0.8)N first cladding layer 21 which was exposed outsidethrough an etch process.

FIG. 10 shows optical-output characteristics of the obtained ultravioletlight-emitting diode. The comparison of emission wavelength and voltageduring an operation at 50 mA was as follows: 323 nm→7.4V, 338 nm→7.1 V,352 nm→5.0 V, and 363 nm→4.7 V. FIG. 10 also shows the test resultsreported in the aforementioned Non-Patent Publications 1 and 3 ascomparative examples 1 and 2, respectively. As seen in FIG. 10, theinventive example exhibits an external quantum efficiency of 0.1% ormore in the emission wavelength range of 370 nm or less, which is higherthan that of the comparative examples, at any emission wavelength.

INDUSTRIAL APPLICABILITY

The present invention can be used to produce a group-III nitridesemiconductor substrate having a low dislocation density in the entiresurface. Thus, the present invention can achieve high-performanceshort-wavelength light-emitting and light-receiving devices.

1. A nitride semiconductor substrate for a device emitting a shortwavelength light of 370 nm or less comprising: a single-crystalsubstrate; a first semiconductor layer made of (0001) faceAl_(x)Ga_(1-x-y)In_(y)N (0≦x≦0.1, 0≦y≦0.5); a plurality of seed crystalseach made of Al_(x)Ga_(1-x-y)In_(y)N (0≦x≦0.1, 0≦y≦0.5) and formed onsaid semiconductor layer to have a slant surface while being arranged inan equally-spaced stripe pattern; an intermediate layer made ofAl_(a)Ga_(1-a-b)In_(b)N (0.1≦a≦1, 0≦b≦1), and deposited on a wholesurface of said seed crystals at low temperature of 300 to 800° C.; anda second semiconductor layer made of Al_(a)Ga_(1-a-b)In_(b)N (0.1≦a≦1,0≦b≦1), and formed on said intermediate layer to have a flat surface. 2.The nitride semiconductor substrate as defined in claim 1, wherein saidslant surface is (1-101) face or (11-22) face.
 3. The nitridesemiconductor substrate as defined in claim 1, wherein saidAl_(x)Ga_(1-x-y)In_(y)N is GaN.
 4. The nitride semiconductor substrateas defined in claim 1, wherein said single-crystal substrate is made ofone selected from the group consisting of sapphire, silicon carbide,silicon and ZrB₂.
 5. A method of producing a nitride semiconductorsubstrate comprising the steps of: performing a selective growth processto form a plurality of seed crystals made of Al_(x)Ga_(1-x-y)In_(y)N(0≦x≦0.1, 0≦y≦0.5) on a first semiconductor layer made of (0001) faceAl_(x)Ga_(1-x-y)In_(y)N (0≦x≦0.1, 0≦y≦0.5), in such a manner said seedcrystals are provide, respectively, with slant surfaces, and arranged inan equally-spaced stripe pattern; depositing an intermediate layer madeof Al_(a)Ga_(1-a-b)In_(b)N (0.1≦a≦1, 0≦b≦1), on said seed crystals atlow temperature of 300 to 800° C.; and growing a second semiconductorlayer made of Al_(a)Ga_(1-a-b)In_(b)N (0.1≦a≦1, 0≦b≦1), on saidintermediate layer until said second semiconductor layer has a flatsurface.
 6. A nitride semiconductor substrate comprising: asingle-crystal substrate having equally-spaced stripe-shaped grooves; aplurality of seed crystals each made of Al_(x)Ga_(1-x-y)In_(y)N(0≦x≦0.1, 0≦y≦0.5) and formed on the convex portions of saidsingle-crystal substrate to have a slant surface; an intermediate layermade of Al_(a)Ga_(1-a-b)In_(b)N (0.1≦a≦1, 0≦b≦1), and deposited on saidseed crystals at low temperature of 300 to 800° C.; and a secondsemiconductor layer made of Al_(a)Ga_(1-a-b)In_(b)N (0.1≦a≦1, 0≦b≦1),and formed on said intermediate layer to have a flat surface.
 7. Thenitride semiconductor substrate as defined in claim 6, wherein saidslant surface is (1-101) face or (11-22) face.
 8. The nitridesemiconductor substrate as defined in claim 6, wherein said seedcrystals are arranged in an equally-spaced stripe pattern, and made ofGaN.
 9. The nitride semiconductor substrate as defined in claim 6,wherein said single-crystal substrate is made of one selected from thegroup consisting of sapphire, silicon carbide, silicon and ZrB₂.
 10. Amethod of producing a nitride semiconductor substrate comprising thesteps of: forming a plurality of seed crystals each made ofAl_(x)Ga_(1-x-y)In_(y)N (0≦x≦0.1, 0≦y≦0.5), on a single-crystalsubstrate having equally-spaced stripe-shaped grooves, in anequally-spaced stripe pattern; depositing an intermediate layer made ofAl_(a)Ga_(1-a-b)In_(b)N (0.1≦a≦1, 0≦b≦0.5), on said seed crystals at lowtemperature of 300 to 800° C.; and growing a semiconductor layer made ofAl_(a)Ga_(1-a-b)In_(b)N (0.1≦a≦1, 0≦b≦1), on said intermediate layeruntil said semiconductor layer has a flat surface.
 11. A semiconductorlight-emitting device comprising: the nitride semiconductor substrate asdefined in claim 1 or 6; a first conductive-type cladding layer made ofAl_(x)Ga_(1-x-y)In_(y)N (0.1≦x≦1, 0≦y≦0.5); a second conductive-typecladding layer made of Al_(x)Ga_(1-x-y)In_(y)N (0.1≦x≦1, 0≦y≦0.5); andan active layer made of Al_(x)Ga_(1-x-y)In_(y)N (0≦x≦1, 0≦y≦1), saidactive layer having a bandgap less than that of said cladding layers.12. The semiconductor light-emitting device as defined in claim 11,which has an emission wavelength of 370 nm or less, and an externalquantum efficiency (η ext) of 0.1% or more, wherein said externalquantum efficiency (η ext)=Po/(I×V), wherein Po is a light output, Ibeing an operating current of said device, and V being an operatingvoltage of said device.
 13. A photodetector comprising: the nitridesemiconductor substrate as defined in claim 1 or 6; a firstconductive-type semiconductor layer made of Al_(x)Ga_(1-x-y)In_(y)N(0.1≦x≦1, 0≦y≦0.5); a second conductive-type semiconductor layer made ofAl_(x)Ga_(1-x-y)In_(y)N (0.1≦x≦1, 0≦y≦0.5); and a light-absorbing layermade of Al_(x)Ga_(1-x-y)In_(y)N (0≦x≦1, 0≦y≦1).